1. Field of the Invention
The present invention relates to a semiconductor device which has a circuit including a thin film transistor (hereinafter referred to as a TFT) and a method for manufacturing the semiconductor device. For example, the present invention relates to electro-optic devices typified by display devices using a liquid crystal element or a light emitting element, and to electronic devices mounted with those electro-optic devices as a component.
Note that a semiconductor device in this specification refers to all electrical devices which can function by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic appliances are all semiconductor devices.
2. Description of the Related Art
Most of display devices such as display of liquid crystal televisions, personal computers, and cellular phones that are widely available use liquid crystal display devices utilizing TFTs using amorphous semiconductor including silicon as a main structural element for a switching element. A TFT using an amorphous semiconductor (hereinafter, referred to as an amorphous semiconductor TFT) is conventionally formed to have a layered structure by a photolithography process using five photomasks. Further, in recent years, using multi-tone photomasks, a process of manufacturing amorphous semiconductor TFTs using photomasks, the number of which is reduced to four has been developed.